The present invention relates to an actuator device. Furthermore, the present invention relates to a process for producing an actuator device.
It is known from the prior art to use magnetic shape memory (MSM) alloy materials for actuator engineering. For this purpose, as shown schematically in FIG. 19 relating to the supposed prior art, an MSM crystal body (as a representative of a multiplicity of MSM-based materials, for instance foams, polycrystals, composites, etc. and bodies to be manufactured therefrom), typically realized on the basis of an NiMnGa alloy, is exposed to a magnetic field. In the schematic illustration in FIG. 19, in this respect the MSM crystal body 10 shown in elongated form is suitably held between a pair of solenoids 12, which, as illustrated by the line illustration 14, generate a horizontally running field and act on the body 10 therewith. As a reaction to the exposure to a magnetic field, the crystal body 10 performs an expansion movement in the arrow direction 16, and, in a specific application, may interact with an appropriately coupled actuating partner.
Therefore, magnetic shape memory alloy materials of this type and actuator devices realized therewith offer an interesting possibility for replacing or for complementing common actuator principles (such as electromagnetic actuators); in addition to basic simplicity of mechanical design in realizing such devices (no armature moves as a whole, merely an expansion of a body takes place), an advantage of the magnetic shape memory alloy principle used is primarily a potentially quick reaction time of the expansion to the application of a magnetic field of the required strength; in addition, depending on the configuration, actuating forces which are already sufficient for the current state of the art technology for many application purposes can be generated.
Nevertheless, such actuator devices, which are assumed to be known in principle, also entail disadvantages (owing to the principles and construction involved) which to date have restricted a truly universal applicability of such actuators. Thus, for example, a utilizable stroke of the expansion movement (i.e. a degree of elongation of an elongation movement performed by the actuator crystal) is typically limited to approximately 3 to 6% of a corresponding axial extent of the crystal, such that particularly large-stroke movements can only be realized with difficulty by means of shape memory alloy actuators, and therefore there is a need to extend this expansion stroke or to optimize it as far as possible.
In addition, typical shape memory alloy materials have the property that the intended expansion movement takes place as a reaction to a magnetic field (of an appropriate minimum field strength), but after a decline in the magnetic field below this threshold, compression does not then automatically take place back into the original compressed state of the expansion unit realized by means of the shape memory alloy crystal. Instead, the crystal remains in the expanded position even in the event of a decline to below an expansion threshold or in the event of complete deactivation. It has therefore been discussed in the case of devices known from the prior art to realize the restoring (i.e. the resetting of the expansion to the non-expanded starting position) with restoring means which either themselves have a shape memory alloy actuating element (expanding in a correspondingly opposite direction), or alternatively such a restoring device exerting a restoring force in the restoring direction and therefore counter to the expansion direction. If the spring force of such a restoring spring is set, with respect to an (unstressed) expansion force of the shape memory alloy crystal, in such a way that the expansion force exceeds the spring force when the shape memory alloy material is subjected to a magnetic field, the intended expansion movement takes place. In the event that the magnetic field declines, and the expansion force accordingly reduces, the spring force is then above the expansion force, however, and accordingly resets the crystal into the contracted starting position thereof.
This mechanism of action is illustrated on the basis of FIG. 20 relating to the supposed prior art. A shape memory alloy expansion crystal, for instance of a type shown in accordance with FIG. 19, shows, in the force-travel(-stroke) graph shown in FIG. 20, a stroke profile (expansion force profile) as is illustrated in the top curve 20. It can be seen that, with an approximately constant negative pitch, this expansion force profile extends up to approximately 0.9 mm stroke, and then declines steeply. The lower characteristic curve, as a restoring characteristic curve 22, shows the input of force which is required for restoration (resetting) counter to the stroke travel in a state of the crystal in which it is not subjected to a magnetic field.
The shape memory alloy crystal actuator shown in FIG. 20 with reference to the characteristic curves 20, 22 is combined with a restoring spring, which, realized in the form of a typical helical spring, and in a manner which is not shown in the figures, interacts at the face with the crystal 10 and, counter to the arrow direction 16 (FIG. 19), exerts a restoring force on the crystal which is set by the restoring characteristic curve 24, in accordance with a Hooke's straight line 24 which is to be assumed to be potentially idealized.
From the points of intersection between said spring characteristic curve (a typical pitch to be applied in the present stroke range is approximately 5 N/mm) and the stroke characteristic curve 20 or the restoring characteristic curve 22 leads to the effective movement or stroke range, which is limited between a lower stroke limit 26 and an upper stroke limit 28, of the shape memory alloy actuator shown by way of example. Within this range, sufficiently subjecting the crystal to a magnetic field firstly ensures the provision of an actuating force which exceeds the spring force and is therefore sufficient for driving an actuating partner, and at the same time the restoring force of the spring makes it possible, after deactivation of or a decline in the magnetic field, to reset (compress) the expansion body into the starting position thereof within the stroke range slightly above 0. The maximum stroke which thus arises is therefore approximately 0.8 mm in the example.
The graph of FIG. 20 additionally shows, by means of the shaded area 30, the effective expansion work which results from the interaction of the expansion unit (when subjected to a magnetic field) and a restoring force (acting counter to the latter), this being described as the integral of the difference in force of both partners over the effective expansion stroke (i.e. the region between the portions 26 and 28). It not only becomes apparent that this difference in force used for an actuation behavior decreases continuously in the extended (right-hand-side) expansion range, and in this respect provides an actuating partner with increasingly less drive force in the direction towards the expansion direction, but in addition for instance a comparison of the effective expansion work (area 30) relative for instance to the hysteresis difference between the stroke characteristic curve 20 and the restoring characteristic curve 22 in the expansion range (and beyond) shows that only a fraction of the expansion work made possible by the shape memory alloy body can be used, as a result of the interaction with the restoring spring.